The present invention is directed to the use of trees having their lignin content and structure modified so that the wood becomes plastic or is more compatible with plasticizing agents. The wood may be molded, extruded, or otherwise treated under heat and pressure to form products conventionally useful as lumber or moldings or it may be formed into many products currently made from molded plastics, composites, or inorganic materials.
Over the many millennia of human history and prehistory, trees have been one of natures most valuable and useful products to man. Before modern civilization, the wood that forests provided has been a material for heat, shelter, drugs, flavorings, art, weapons, and even clothing. In more recent times it has additionally had incalculable importance as a source of paper for its myriad uses in communication, personal hygiene care, packaging, and many other products.
Forest types vary over the globe but they may normally be classified as those in which conifers or deciduous trees predominate. The conifers, or so-called softwoods, are the source of most of the world""s construction lumber and much of its paper. Deciduous trees, typically known as hardwoods, are much used where appearance is important, as well as for paper products where shorter fibers give advantageous properties. Many deciduous tree species have extremely beautiful wood from the standpoint of color and/or grain pattern. The term xe2x80x9chardwoodxe2x80x9d is actually a misnomer since actual hardness spans a wide range from extremely soft to very dense. Also, many deciduous trees from tropical or semi-tropical areas do not seasonally loose their leaves as they do in temperate zones. On the scale of evolutionary development, the deciduous trees have appeared much more recently in geologic time than conifers and are considered to be more advanced. The separation occurs at a high level on the taxonomic scale with the deciduous trees being in the botanical Class Angiospermae while the conifers are encompassed in the Class Gymnospermae. As would be expected, this wide evolutionary separation has resulted in significant differences in morphology and wood chemistry:
If the coniferous woods may be used as a model, they are composed of longitudinal fibers (tracheids) with a much lower number of thin-walled, radially oriented ray parenchyma cells. The tracheids, which are typically 2-5 mm long, are closed at the ends and have a central hollow or lumen extending most of their length. The tracheid walls have multiple layers with a multiplicity of openings (simple or bordered pits) through the walls into the lumen and in communication with similar openings in the adjacent tracheids and parenchyma cells. A layer called the middle lamella is located in the intermediate zone between adjacent tracheids. Structure of the xylem, or woody tissue, of the hardwoods has all of these features with the addition of other functional cells such as large, longitudinal thin-walled vessels.
As is well known, the primary structural and chemical constituent of the tracheids is the polymer cellulose. This occurs in the tracheid walls along with lower molecular weight polymeric sugars (hemicellulose) of somewhat different linkage and composition. In angiosperms the hemicellulose is a mixture of complex hexose and pentose polymers. Gymnosperm hemicellulose has a lower content of the pentose polymers. The tracheid walls are further reinforced with complex heterogeneous aromatic-based polymers referred to as lignin. The lignin is believed to contribute mechanical stiffness and structural integrity to the standing tree. The middle lamella between the tracheids is an amorphous zone composed primarily of hemicellulose and lignin. Middle lamella lignin may or may not be of similar composition to that in the tracheid walls. Fergus and Goring, Holzfosrchung 24(4): 113-117 (1970) note that birch lignin in the vessel secondary wall and middle lamella is predominantly composed of one type (guaiacyl) whereas lignin in fiber and ray parenchyma secondary walls is mainly of another type (syringyl). The middle lamella around the fiber and ray cells contained both types.
Lignin is formed during cell development by the sequential expression of several known genes. Lignin formation and composition has been extensively studied and there is an extensive literature on the subject. At some point after the evolutionary separation of the gymnosperms and angiosperms the composition of the lignin followed different paths. Lignin polymers are generally classified into three groups depending on their respective monomer units. The conifers have predominantly crosslinked guaiacyl-type lignins formed as dehydrogenation polymers of coniferyl alcohol. In contrast, hardwood lignins are composed of a roughly equal mixture of guaiacyl and the more linear syringyl-types, the latter being a dehydrogenation polymer of sinapyl alcohol. A third type, guaiacyl-syringyl-p-hydroxyphenyl is found in grasses. These are believed to be the most evolutionarily advanced of the group; e.g., see T. Higuchi et al. Wood Science Technology 11: 153-167 (1977). These authors trace the entire biosynthetic pathways of lignin formation in both gymnosperms and angiosperms. Briefly stated, phenylalanine is first deaminated to produce cinammic acid. This is then methylated and hydroxylated to produce the three acids basic to synthesis of the three lignin types. These are then reduced to aldehydes by the enzyme cinnamoyl-CoA reductase (CCR) and finally converted to alcohols (monolignols) by another enzyme cinnamyl alcohol dehydrogenase (CAD). Subsequent polymerization occurs principally at the beta carbon of the propanoid moiety and at C5 of the aromatic ring, although other point of reactivity are also involved. 
Lignin genesis and chemistry and the history of its investigation is concisely reviewed by E. Adler, Wood Science and Technology 11 169-218 (1977). Sarkanen and Ludwig treat the subject in great depth in Lignins. Occurrence, formation, structure and reactions, Wiley-Interscience, New York (1971).
The difference in lignin composition is of far more than academic importance. Guaiacyl lignins tend to be far more heavily crosslinked than syringyl types. This affects their relative solubility in the various pulping liquors used for preparation of wood pulps. The syringyl lignin, being a more linear polymer due to one less available crosslinking site, is more readily solubilized and removed by the usual pulping chemicals. All other things being equal, hardwood species are more easily pulped than coniferous species.
Recent research had been directed to finding or creating both coniferous and hardwood trees that have modified lignin more amenable to removal by conventional pulping processes. This research has followed two lines. One is classic genetic selection in which trees having desirable properties are selected and reproduced. The other is genetic transformation in which new genes are introduced into a species by one of the techniques now available.
An example of the genetic selection route is detailed in MacKay et al., U.S. Pat. No. 5,824,842. In a case of serendipitous research, a scientist noted a loblolly pine (Pinus taeda L.) which had brownish sap wood in comparison to the usual white wood of the species. Further research revealed that the tree had a mutant gene that failed to produce the enzyme cinnamyl alcohol dehydrogenase (CAD). This enzyme lies on the critical lignin synthesis path for the guaiacyl lignins, acting to convert coniferaldehyde to coniferyl alcohol. The latter of these compounds is the primary lignin precursor in conifers. Although it had been harvested, fortunately the tree was in a research plot and had known parentage with the maternal parent still available as a seed source. Further plantings and crosses originating from the parental seed resulted in trees that were both heterozygous and homozygous in the mutant CAD null allele. Wood from both heterozygous and homozygous trees has been pulped on a micro scale using both the soda and kraft processes; e.g., see MacKay et al., Holzforschung 53: 403-410 (1999). The authors concluded thatxe2x80x9c . . . suppression of CAD in softwood trees may hold promise to produce woods well suited for xe2x80x98milderxe2x80x99 pulping conditions that consume less chemicals . . .xe2x80x9d. Lignin composition in the CAD deficient trees was significantly modified in comparison to that found in a normal tree; e.g., see Ralph et al. Science 277: 235-239 (1997). Guaiacyl lignin was very low and there were increased amount of coniferaldehyde, dihydroconiferyl alcohol, and vanillin and decreased amounts of coniferyl alcohol. The authors particularly note the approximate tenfold increase in dihydroconiferyl alcohol, a monomer not usually associated with the biosynthesis of lignin. This is of considerable practical importance since dihydroconiferyl alcohol lacks the double bond on the propanoid moiety and thus lacks the reaction site at this location. It can form only linear polymers.
Baucher et al., Plant Physiology 112: 1479-1490 (1996) report easier lignin extractability in a poplar cross by down-regulating CAD. The amount of lignin in juvenile stems was not decreased nor was the syringyl/guaiacyl ratio significantly changed. However, kraft pulping tests indicated greater lignin removal as evidenced by reduced kappa numbers. Similarly, Boudet et al, in U.S. Pat. No.5,451,514 describe poplar and eucalyptus genes and recombinant DNA containing the genes useful for reducing the level of CAD in a plant. Bloksberg et al. in U.S. Pat. No. 5,850,020 describe genetic modification of lignin in a wide variety of plants including poplar and eucalyptus.
It has long been known that some species of wood can be bent when thoroughly steamed and while still hot. Birch and ash species are exemplary. Bent wood is used extensively in furniture, especially in items such as chair backs. In general, conifers are very difficult to bend without failure on the tension side of the curve. A high wood moisture content is essential and it is well known that moisture serves as a wood plasticizer. Apparently some plastic flow occurs during bending. The U.S. Forest Products Laboratory has developed a heat stabilized compressed wood (Stapak) in which they refer to lignin xe2x80x9cflowxe2x80x9d under pressure, although no supporting evidence is given; e.g., Seborg et al. Forest Products Laboratory, Forest Service, U.S. Dept. of Agriculture Publication 1580 (1956). Baldwin and Goring, Svensk Papperstidning 71(18): 648-650 (1968) describe the thermoplastic and self adhesive behavior of birch, aspen, and black spruce fiber after steam treatment prior and subsequent to mechanical defiberization. They note that the respective softening temperatures for dry hemicellulose and lignin of 160xc2x0-200xc2x0 C. drop markedly when moisture is present.
Bending or compression of wood is primarily a two dimensional deformation. Efforts to mold or otherwise form wood in three dimensions have had very limited success. Yet an analogy might be made between wood and a reinforced plastic. In wood the tracheids form a highly oriented reinforcing fiber and the middle lamella serves as the matrix in which the fibers are embedded. If sufficient flow could be created in the middle lamella it should be possible to form wood into three dimensional and other forms by inexpensive molding rather than by machining as is now required. Since wood is basically a very low cost material, major economic advantages would be realized if it could be substituted in some applications for the much more expensive plastic materials. As one example, the CAD deficient pine noted earlier appears to have a much higher content of lower molecular weight products and is possibly less crosslinked, suggesting that it might be thermoformable. However, efforts at lignin modification to date have been directed to producing more readily pulpable woods. The advantage of the increased plasticity of modified lignin woods has been totally unrecognized to the present time.
It has been discovered by the present inventors that that the wood of certain trees having naturally or genetically modified lignin has plastic properties, or can be readily plasticized, so that the wood has at least limited flow under appropriate conditions of pressure and temperature. These plastic properties are significantly greater than the wood of similar species trees having normal lignin composition. The wood of trees in which the lignin is relatively uncrosslinked, of lower molecular weight, of a relatively higher content of the monomers and oligomers of lignin percursors, or is readily extractable by the usual methods, has been found to exhibit enhanced plastic behavior. Trees having a relatively high content of syringyl-type lignin and low amounts of guaiacyl lignin, especially in the middle lamella, are among those having this plastic flow. This bias toward modified lignin formation is typical of some plant species having a genetic modification producing a deficiency of the enzyme cinnamyl alcohol dehydrogenase (CAD). It may be assumed that trees having other genetic modifications producing lignins of greater molecular linearity and/or lower molecular weight would have similarly valuable plastic properties. An increased content of these generally lower molecular weight or less crosslinked lignins, especially in the middle lamella region, is conducive to greater plasticity.
The wood of these modified lignin trees having plastic properties can be molded or formed into more complex shapes than has heretofore been possible with normal wood. The terms xe2x80x9cmoldedxe2x80x9d or xe2x80x9cformedxe2x80x9d are used in the context that an original shape or configuration may be shaped into a different permanent and useful configuration by application of pressure at appropriate temperatures. Depending on the nature of the ultimate product, the molded piece may be formed from an original single or unitary piece of wood. Alternatively, it may be formed from a plurality of pieces or particles of wood which are then formed into a unitary piece by application of pressure. True plastic flow has been observed to occur during the molding process. The wood may be comminuted into particles, flakes, strands, or other geometric configurations prior to molding. Additionally, plasticizers or other adjuvants may be added to the modified lignin wood prior to molding. Plasticizers may be those that interact with either lignin or cellulose to improve processability or to modify certain physical or mechanical properties of the wood. For a chemical to function as a plasticizer for a polymer it should have a solubility parameter close to that of the polymer. Chemical agents having solubility parameters close to cellulose, hemicellulose or lignin, most preferably close to all three, are expected to function as effective wood plasticizers. Exemplary effective plasticizers are water, carbon dioxide, sulfur dioxide, dimethylsufloxide (DMSO), dimethylformamide (DMF), ammonia, formaldehyde, urea and ureaformaldehyde condensates, phenols, and phenol-formaldehyde monomers and oligomers. Many chemicals capable of forming hydrogen bonds with lignin are also effective plasticizers. As examples, chemicals with substituents chosen from amines, alcohols including polyols, ketones, carboxylic acids, esters, ethers, amides, isocyanates, nitriles, nitrates, thiols, thio esters, sulfonic acids, sulfonates, sulfoxides, and sulfones can, under certain conditions, serve as wood plasticizers. Many of the above plasticizers can be removed and recovered, if desirable, after formation of the product.
It is within the scope of the invention to include various types of materials which may act as adhesives or as both plasticizers and adhesives among the adjuvants. Adjuvants also include materials that can act as reinforcement or fillers including glass or metal fibers and mineral fillers of which clays, carbonates, and metal oxides would be exemplary.
In one application, the wood is formed into discrete particles that are prepared as a preform or simply placed in a mold and subjected to sufficient pressure at a suitable temperature for an adequate time to prepare a molded product. The wood particles here may be considered analogous to a conventional plastic molding powder. The mold may be a flat platen press or a compression mold. The particles may also be extruded into a linear or planar form. In another application, the lignin modified wood is first formed into elongated strands or splinters. These may be laid up so that the strands are parallel to each other and molded or formed into flat panels or lumber-like products. In this case the fiber orientation is parallel giving the advantage of enhanced strength and stiffness in bending. Sufficient plastic flow occurs so that the particles are permanently joined together, normally without the need for an adhesive.
In a further application, the lignin modified trees may be sawn into conventional lumber. This may then be finished by application of sufficient pressure and heat to produce a smooth surface without significant compression of the interior portion of the lumber piece. In essence, the piece is finished by a process analogous to ironing rather than by planing. One way this is accomplished is by forcing the rough sawn piece through a finishing die. The surfaces may or may not be initially treated with a plasticizer prior to the smoothing treatment. Treatments of this type open the door to significantly less loss into low value or waste products such as chips, planer shavings, or saw dust.
It is within the scope of the invention to selectively densify areas of a wood product to increase strength in zones that will be subjected to the highest stress in use.
Veneers from the lignin modified trees may be pressed into more complex three dimensional shapes, without the usual problem of tearing, than has heretofore been possible.
It is an object of the invention to provide a lignin modified wood having plastic properties for forming into shaped articles.
It is a further object to provide a method for forming wood having plastic properties into shaped molded articles.
It is another object to form lignin modified wood comminuted into particles or strands into unitary articles without the need for adhesives or with only low levels of adhesives.
It is also an object to form wood into predetermined shapes by extrusion.
It is still a further object to provide finished lumber or moldings with lower raw material loss.
These and many other objects will become readily apparent upon reading the following detailed description taken in conjunction with the attached figures.